This invention relates generally to the treatment of the surfaces of devices, especially semiconductor wafers and other electronic or electro-optical devices, at various stages of production. It relates more particularly to novel cleaning, chemical treatment and drying processes in which, instead of a condensed phase medium (liquid), a foam is used as a medium for the various operations such as cleaning, etching, neutralization and drying.
Semiconductor cleaning, chemical treatment and drying technology is well developed. Examples of known processes are those described in U.S. Pat. No. 4,781,764, dated Nov. 1, 1988, U.S. Pat. No. 4,911,761, dated Mar. 27, 1990, U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 U.S. Pat. No. 5,656,097, dated Aug. 12, 1997 and U.S. Pat. No. 5,571,337, dated Nov. 5, 1996. However, cleaning, chemical treatment and drying of semiconductors is very expensive. Moreover, performance requirements will soon exceed the present and expected capabilities of current cleaning techniques.
Current processes for the cleaning and treatment of semiconductor wafers and other electronic devices have several serious drawbacks from the standpoint of cost, safety and effectiveness.
High purity deionized water is typically used as a solvent. However, achieving the necessary high purity levels is very expensive. Indeed, all phases of the cleaning operation, including purchasing, transportation, storage, internal distribution, consumption, and disposal, are expensive.
Most of the substances used in the cleaning and chemical treatment processes, such as fluorides, solvents, acids, heavy metals, oxidizers, etc., are toxic, flammable, or otherwise hazardous or obnoxious.
Chemical treatment and cleaning operations are also major sources of chemical contamination of the final product. Such contamination results from errant surface reactants, and physical contamination by undesired, very small solid particles. These very low levels of contaminants are delivered to the product, in part from the chemical treating and cleaning materials themselves, even though they are ultrapure. They are also delivered to the product from fittings, piping, tanks, valves, and other components of storage and delivery systems.
Contaminants on semiconductor wafer surfaces exist as films, discrete particles or groups of particles and adsorbed gases. Surface films and particles can be classified as molecular compounds, ionic materials and atomic species. Molecular compounds are mostly particles or films of condensed organic vapors from lubricants, greases, photo resists, solvent residues, organic components from deionized water or plastic storage containers, and metal oxides or hydroxides. Ionic materials comprise cations and anions, mostly from inorganic compounds that may be physically adsorbed or chemically bonded, such as ions of sodium, fluorine and chlorine. Atomic or elemental species comprise metals, such as gold and copper, which may be chemically bonded to the semiconductor surface, or they may consist of silicon particles or metal debris from equipment.
Semiconductor devices, especially dense integrated circuits, are vulnerable to all of these contamination sources. The sensitivity is due to the small feature sizes and the thinness of the deposited layers on the wafer surface. These dimensions are in the submicron range. The small physical dimensions of the devices make them very vulnerable to particulate contamination in the air, from workers, generated by the equipment, and present in processing chemicals. As the feature size and films become smaller, the allowable particle size must be controlled to smaller dimensions. In general, the particle size should be 10 times smaller than the minimum feature size. Currently, the minimum feature size for commonly available semiconductor chips is 0.25xcexc, therefore suggesting particle control to 0.025xcexc.
Conventional cleaning technologies, utilizing condensed phase solutions, when properly applied, remove a majority of the contaminants generated during the chemical processing of the semiconductor wafers. Liquid systems currently in use can delivery satisfactory results, and acceptable product can be produced. However, the current trend is to require the chemical and equipment suppliers to provide increasingly clean performance. Equipment and chemical suppliers are facing tremendous performance challenges as the feature size decreases. At the same time, semiconductor manufacturers do not want their costs to increase.
Another problem addressed by this invention is the drying of surfaces in the production of semiconductor wafers and similar devices.
Semiconductor wafers are not manufactured in a continuous process. Since there are many semiconductor wafer configurations, batches of wafers are processed through certain steps, and then stored. Later the batches are subjected to additional processing steps, and again stored. The processing and storage sequence may be repeated several times before processing is completed.
In general, at the end of each process sequence, the semiconductor wafers are dried, often even when the next step will proceed almost immediately. Wafers can be transported from one process sequence to the next only after they have been dried, and they can only be stored safely when they are dry. Therefore, the drying process is carried out frequently in the processing of a given wafer, and is very important.
Recently, isopropyl alcohol has become a preferred drying solvent. A variety of processes have been developed and commercialized using isopropyl alcohol either hot or cold, and as a vapor, a liquid or a combination of vapor and liquid. Semiconductor wafer producers have been moving toward reduced isopropyl alcohol usage because of its cost, fire hazards, disposal problems, and VOC (volatile organic compound) emissions.
U.S. Pat. No. 4,911,761, dated Mar. 27, 1990, describes semiconductor wafer processing in which various fluids passed over wafers in fixed positions. The drying sub-system utilizes superheated isopropyl alcohol vapor generated in a distillation apparatus.
U.S. Pat. No. 5,271,774, dated Dec. 21, 1993 describes a technique for removing water from a semiconductor wafer using low levels of solvents such as isopropyl alcohol, applied as a vapor, to reduce the surface tension of a film of liquid on the substrate, and thereby reduce the quantity of material remaining on the surface of the substrate. A centrifuge is used to facilitate the removal of the liquid film.
U.S. Pat. No. 5,571,337 describes another technique for drying semiconductor wafers, utilizing a trace amount of a polar organic compound in a carrier gas composed of oxygen, nitrogen, argon, or mixtures. This patent describes the drying of wafers without the use of isopropyl alcohol, using only warm nitrogen gas. Thus, the industry has proceeded from the use of large quantities of isopropyl alcohol, to minimal quantities of isopropyl alcohol, and then to processes which use no isopropyl alcohol at all.
The principal objects of this invention are to increase the effectiveness of chemical treatment, cleaning and drying operations, and to reduce the cost of such operations. Further objects of the invention are to improve the safety of the chemical treatment, cleaning and drying operations and to reduce the discharge of hazardous or obnoxious substances from the treating and cleaning operations.
Briefly, the invention takes advantage of a desirable characteristic of foam, namely that, from a volumetric standpoint, a given quantity of foam consists mostly of gas. Therefore the quantity of small particles delivered to a substrate by the liquid component of the foam is much smaller than the quantity of particles delivered to a substrate by an equivalent volume of a liquid.
The expansion ratio of foam, i.e. the volume of the foam divided by the volume of its liquid component, defines the cost and performance benefit available from the use of foam. For example, if the expansion ratio is 10, the volume of liquid is reduced to {fraction (1/10)} of the volume of liquid to which the substrate is exposed in a condensed phase chemical treatment or cleaning step. This not only achieves a theoretical materials cost reduction of 90% but also reduces the quantity of contaminating particles delivered to the substrate by a factor of 10. A small increase in the expansion ratio results in a relatively large overall benefit. Additional cost savings can be obtained as a result of the reduction in the volume of the cleaning medium. The resulting smaller inventory of cleaning medium, and the size reductions that can be achieved in components such as tanks, valves, pipes, pumps, etc., lead to lower floor space requirements, which, for semiconductor fabrication facilities, is very significant.
In accordance with one aspect of the invention, the treatment, i.e. cleaning or chemical treatment, of a semiconductor substrate is carried out by the steps of generating a foam consisting of gas bubbles and a liquid component, and causing the foam to pass over the substrate while in moving contact therewith.
In accordance with another aspect of the invention, a semiconductor substrate is dried by a process comprising the steps of generating a foam consisting of carbon dioxide bubbles and deionized water, and causing the foam to pass over the substrate in moving contact therewith.
The cleaning or chemical treatment steps, and the drying step can be carried out in the same treatment vessel. The cleaning, chemical treatment and drying steps can be carried out in sequence without removing the substrate from the treatment vessel.
Other features of the invention are applicable not only to the cleaning, chemical treatment and drying of semiconductor substrates, but also to the cleaning, chemical treatment and drying of other substrates where the removal of extremely small particles or other contaminants, or avoidance of their deposition onto the substrate, is important.
Preferably, the semiconductor substrate is supported in a foam treatment vessel, the foam is generated outside the foam treatment vessel and introduced into the foam treatment vessel, and foam in contact with the semiconductor substrate is caused to pass over the substrate as a result of its displacement by foam introduced into the foam treatment vessel. A sufficient quantity of foam may be introduced to fill the treatment vessel and thereafter, by continued introduction of foam into the treatment vessel, foam may be caused to discharge from the treatment vessel.
For cleaning the substrate, the liquid component of the foam may consist of a surface tension-reducing agent and deionized water, and the movement of the foam removes particles from the substrate. For chemical treatment of the substrate, the liquid component of the foam may include one or more reactants, such as ammonium hydroxide, hydrofluoric acid, nitric acid, etc., so that a chemical reaction takes place between the substrate and the reactant as the foam passes over the substrate. The reactants themselves may serve as surface tension-reducing agents. However, ordinarily, since the reactants will be present in insufficient concentrations to produce adequate quantities of foam, conventional surfactants or other additional surface tension-reducing agents will be included along with the reactants.
In a preferred embodiment, the foam introducing step is carried out by first introducing a foam consisting essentially of a surface tension-reducing agent and deionized water, thereafter introducing a foam comprising deionized water, a surface tension-reducing agent and at least one reactant for chemical treatment of the substrate whereby a chemical reaction takes place between the substrate and the reactant, and thereafter introducing a foam consisting essentially of a surface tension-reducing agent and deionized water, whereby said at least one reactant is rinsed from the substrate.
A foam-based cleaning system in accordance with the invention can effectively substitute for a liquid phase system using sonic energy.
The process may be carried out by alternately introducing foam consisting essentially of a surface tension-reducing agent and deionized water, and foam comprising a surface tension-reducing agent, deionized water and at least one reactant for chemical treatment of the substrate. By carrying out the process in this manner, a series of chemical treatment steps can be carried out on a semiconductor substrate, all using foam as a vehicle for the reactants.
In accordance with another aspect of the invention, for drying the substrate, the foam treatment vessel is preferably located within a pressurizable containment vessel. The substrate is submerged in a solution of carbon dioxide in deionized water under pressure, and thereafter foam is generated by reducing the pressure within the containment vessel. The pressure reduction causes carbon dioxide bubbles to form a foam layer on the surface of the solution. Thereafter the foam layer is caused to pass over the substrate in moving contact therewith by discharging the solution from the foam treatment vessel.
Other objects and advantages of the invention will be apparent from the following detailed description.